This invention relates to a process for producing adhesive film, more particularly, to a process for producing adhesive film by irradiation of actinic light.
Heretofore, adhesive film has been produced by dissolving a high polymer having reactive groups with a crosslinking agent and a crosslinking agent in a large amount of solvent, coating the resulting solution on a film substrate, removing the large amount of solvent with heating and at the same time crosslinking the adhesive composition. The viscosity of the coating solution is generally from several ten centipoises to several thousand centipoises. As the high polymer which is a matrix of adhesive material, it is preferable to use a high polymer having a higher molecular weight, usually a molecular weight of several hundred thousands, considering properties of the adhesive film as mentioned hereinafter. Therefore, in order to make the viscosity of the coating solution suitable for coating, the amount of the solvent to be used amounts to 70 to 90% by weight.
Thus, it has been a great problem to evaporate and scatter such a large amount of solvent at the time of heating from the viewpoints of saving resources and air pollution. In order to solve such a problem, it has been proposed to recover scattered solvent at the time of heating and partly practiced, but the recovering cost is very large and the recovering yield is not sufficient. On the other hand, in order to lessen the amount of the solvent to be evaporated and scattered, it is thinkable to reduce the amount of the solvent in the adhesive composition before coating. But when the molecular weight of the high polymer which is a matrix of adhesive material is several hundred thousands or more, the viscosity of the coating solution is increased by reducing the amount of the solvent, which makes coating process difficult. Therefore, the amount of the solvent can be reduced to 60 to 80% by weight at most.
In order to reduce the amount of the solvent and to lower the viscosity of the coating solution, the molecular weight of the high polymer which is a matrix of adhesive material should be lowered. But when the molecular weight is lowered, internal cohesion strength of the resulting adhesive film is lowered, which results in easily bringing about migration to an adherend at the time of peeling off the adhesive film after the application of the adhesive film for a long period of time. On the other hand, when crosslinking density is enhanced in order to prevent the migration and to supplement the cohesion strength of the adhesive material itself, the glass transition temperature is increased so as to lower adhesiveness.
As mentioned above, there is a limitation in reducing the amount of solvent to be used in conventional processes for producing adhesive composition and adhesive film from the viewpoints of coating processability and physical properties.
On the other hand, there is disclosed in Dutch Pat. No. 6,601,711 a process for producing an adhesive material without using a solvent. According to this process, a monomer (acrylate monomer) which becomes a polymer used as a mattrix of the adhesive material is coated on a filmlike substrate and polymerized on the substrate by irradiation of ultraviolet light. This process is advantageous in that no solvent is used for producing the polymer as a matrix of the adhesive material. But according to experiments of the present inventors, there are many defects in that the kind of monomers used for producing the polymer which is a matrix of the adhesive material is limited, these monomers are easily evaporated and scattered at the time of irradiation of ultraviolet light and quantum yield by the photochemical reaction is very low. Consequently, there arise many problems such as pollution of the working circumstances by the evaporation and scattering of the monomer, pollution of the apparatus such as a lamp by the adhesion and curing of the monomer evaporated and scattered, migration of unreacted monomer remaining in the produced film to an adherend, and the like. When a monomer hardly evaporating and scattering and having low volatility is used or cured material obtained from a monomer having a high quantum yield is used, it is very difficult to obtain sufficient adhesiveness.
The present inventors have studied to solve the problems mentioned above and accomplished this invention.